The present invention relates to electric circuits, and, more particularly, to a circuit device for driving a brushless electric motor and related method.
An important parameter in driving brushless motors for hard disk drives (HDD), or other applications of similar requirements, is the ratio between the speed and the torque. One approach to optimizing this ratio includes driving the windings of a motor independently from one another through as many full-bridges. In this way, it is possible to apply the full supply voltage to each winding. The windings can therefore be designed to obtain a greater torque and/or rotating speed compared to a traditional motor using phase windings pre-connected in a star or polygon configuration.
In a three-phase motor (i.e., having three windings), to realize a xe2x80x9csix wirexe2x80x9d driving scheme the integrated driver device should have six pins to be connected to the respective terminals of the three windings, as shown in FIG. 1. In the case of xe2x80x9csinusoidalxe2x80x9d driving, the driving system will generate three sinusoids, each 120 degrees out of phase from the others. The relative inverted and direct signals are produced on the six outputs or pins of the integrated device. The current will attain its peak value (Imax) in a phase winding, while the current in the other two phase windings is half the peak value (Imax/2), as shown in FIG. 2.
Should the same integrated driver device be used for driving a traditional motor with phase windings pre-connected in a star or polygon configuration (a triangle in the case of a three-phase motor), such as a three-phase motor in a star configuration (three wires), only three of the six output pins will be connected to the motor (for the considered example A1, B1 and C1), as shown in FIG. 3. In this case the available limit peak current would be the same, thus the power supplied to the motor will be halved because the maximum voltage applied on each winding is halved.
For delivering the same electric power using the same integrated device, the outputs may be connected in parallel to feed the motor with a doubled peak current Imax. This may be done by connecting A1 and A2 to the phase A winding of the motor, B1 and B2 to the phase B winding, and C1 and C2 to the phase C winding, and configuring the driving circuit to phase the A2, B2 and C2 outputs depending on the value of a register of the integrated driving system. This scheme is shown in FIG. 4. Such a configuration is relatively easy to implement.
FIG. 5 shows a typical partial layout of the output half-bridges. The figure also highlights the twelve MOS transistors that make up the three full-bridges, the supply and ground pads, and the relative pins for the connection to the three-phase motor to be driven with independently fed windings. The metal lines that connect the various ground and supply pads to the relative current terminals of the power transistors are normally designed for the expected maximum peak value envisioned for driving a six wire three-phase motor.
The above adaptation for driving a traditional three-phase motor may cause an overload of the metal lines of the integrated circuit; that is, of the output stages toward the respective pins. The doubling of the currents supplied to the motor may cause electro-migration phenomena in the metalizations of the integrated circuit.
As a consequence, an integrated device designed for an independent driving of each winding would normally be unusable for driving an equivalent motor in a star or polygonal configuration at the same power level. This is because of the circuitry-induced doubling of the peak current in the metal supply and ground lines. On the other hand, it would not be cost effective to design an integrated circuit with metal lines of deliberately augmented widths simply because the same integrated circuit may be needed to implement the above adaptation.
It is therefore an object of the present invention to provide a method and a circuit arrangement which allows the driving of a conventional brushless motor having its phase windings pre-connected in a star or polygon configuration by an integrated driving system designed for separately driving each winding of a multi-phase brushless motor. The metal supply and ground lines are correctly sized for separately driving the windings at the same power level without overloading the metal lines toward the supply and ground output pins.
This significant result is obtained by connecting each terminal of the brushless motor having its phase windings pre-connected in a star or polygon configuration to an output pin of the integrated driving system relative to a first phase driving signal and to another output pin relative to a different phase driving signal. An internal configuring means or circuit is provided including a plurality of integrated path deviators or selectors that may be set in one or another position depending on the actual mode of use of the integrated device. The paths of the direct signals to the plurality of output pins are coherently crossed from the external connecting scheme of each terminal of the motor to the output pins of the integrated device. In this way, overloading on the output metal tracks is effectively avoided when a star or polygon configured motor is driven at the same power as in a phase independent driving mode of the windings through respective full-bridges.